1. Field of the Invention
This invention is directed to the field of non-destructive testing of magnetic media for use in disc drives, and more particularly measurement of coercive force and remanence-thickness product as a function of circumference and/or radius of the recording member. The data can be used to adjust process parameters in manufacturing magnetic recording members.
The recording performance of a magnetic recording member is primarily determined by the magnetic properties, specifically the coercive force, remanence, and thickness of the recording member. These magnetic properties determine the signal amplitude, frequency response, resolution, and overwrite characteristics of recording disc members.
Currently, there are two common, methods for measuring these properties, a vibrating sample magnetometer and a hysteresis tester. The former measures the magnetization as a function of the applied field, whereas the latter measures the flux density as a function of field. In order to obtain the fundamental magnetic properties using this equipment, namely, the coercive force, remanence, or remanence-thickness product, as well as the saturation magnetization, it is necessary to cut or define a finite element from the recording member for measurement purposes. Consequently, both techniques are destructive. In addition, both techniques or measurement methods determine the average value of the coercive force, and/or the remanence of the sample, or the finite element which was removed from the recording member for evaluation rather than the values for a specific location(s) on the surface of the member. In addition, such methods are not practical for thin film media typically utilized in advanced digital recording systems due to inadequate magnetic volume.
A typical hysteresis loop or B-H loop is shown in FIG. 1, which describes the typical hysteresis characteristics required for evaluation of a sample. Typical digital recording media consists of magnetic oxide (gamma iron oxide) dispersed in a non-magnetic organic binder.
Thin film media are known to exhibit superior recording performance relative to particulate oxide media as a result of inherent higher remanence and coercive force values. However, the magnetic properties of oxide and thin film media are determined to a large degree by the quality of the particulate dispersion, degree of orientation of the oxide particulate particles, and the particle size. The magnetic properties of thin film media are determined by the deposition process parameters, e.g., in the case of sputtered films, process temperature and substrate type are of particular significance, as well as the sputtering pressure. Consequently, the magnetic properties of a recording member are determined in situ or after the fact. Currently, manufacturing must rely on the recording performance to monitor progress and/or adjust process parameters to conform to recording specifications.
Therefore, a need exists for a method of quickly determining the magnetic properties, particularly coercive force and the remanence-thickness product, in a non-destructive manner.
An object of this invention is to provide a method and apparatus for measuring the coercive force as well as the remanence-thickness product in a non-destructive manner.
A related objective of this invention is to provide method and apparatus for determining the values of the above parameters at any location on the disc surface with a minimum degree of averaging or minimum spatial resolution. This provides precise measurement of the coercive force and remanence-thickness product over the disc surface, which is necessary for determination of the recording performance and manufacturing process control.